1. Field of the Invention
The present invention relates to an optical modulator, a mounting substrate of an optical modulator, and a driving method of an optical modulator. In particular, the invention relates to an optical modulator that is driven differentially, a mounting substrate of such an optical modulator, and a driving method of such an optical modulator.
2. Description of Related Art
FIG. 9 shows a structure of a conventional electroabsorption (EA) modulator. The EA modulator utilizes the electroabsorption effect that an optical waveguide having a pn junction absorbs light when an electric field is applied to it. Having a high response speed, the EA modulator is used as an optical modulator for optical communication.
In FIG. 9, reference numeral 1 denotes an InP substrate; 6, a transparent waveguide that is formed on the InP substrate 1; 25, an EA modulator that is connected to the transparent waveguide 6; 30, a p-type electrode pad of the EA modulator 25; 31, an n-type electrode pad of the EA modulator 25; and 5, an n-type ohmic contact layer that is a matching resistor of the EA modulator 25.
To apply a modulation signal voltage to the EA modulator 25, a driver amplifier outputs two kinds of signals, that is, a positive-phase signal and an opposite-phase signal that is opposite in phase to the positive-phase signal. There are two methods for driving the EA modulator 25. The method in which only one of the positive-phase signal and the opposite-phase signal is used is called single-phase driving. The method in which both of the positive-phase signal and the opposite-phase signal are used is called differential driving.
FIG. 10 shows an equivalent circuit in a case where the EA modulator 25 of FIG. 9 is driven by the single-phase driving. The elements in FIG. 10 that are the same as elements in FIG. 9 are given the same reference numerals as the latter and hence may not be described. In FIG. 10, reference numeral 20 denotes a matching resistor that is connected in parallel to the EA modulator 25; 26, an EA modulator driver for driving the EA modulator 25; 27, a positive-phase signal terminal of the EA modulator driver 26 that outputs a positive-phase signal; 28, a ground terminal of the EA modulator driver 26; and 29, an opposite-phase signal terminal of the EA modulator driver 26 that outputs an opposite-phase signal.
FIG. 10 shows an example in which a positive-phase signal is applied to the EA modulator 25 from the positive-phase signal terminal 27. However, a positive-phase or opposite-phase signal can be applied to one end (L-end or M-end) of the EA modulator 25. The matching resistor 20 (e.g., R=50 Ω) is connected in parallel with the EA modulator 25. Terminal B is grounded.
FIG. 11 shows an equivalent circuit in a case where the EA modulator 25 of FIG. 9 is driven by differential driving. The elements in FIG. 10 that are the same as elements in FIG. 9 are given the same reference numerals as the latter and hence may not be described. As shown in FIG. 11, a positive-phase signal and an apposite-phase signal are applied to the two ends (L-end and M-end) of the EA modulator 25. Two resistors 20 (e.g., 2R=50 Ω+50 Ω) are connected in parallel with the EA modulator 25. The connecting point (terminal N) of the two resistors 20 is grounded.
In the differential driving shown in FIG. 11, the voltage that can be applied to the EA modulator is two times higher than in the single-phase driving shown in FIG. 10. In general, a larger extinction ratio can be obtained when the voltage that can be applied to the EA modulator 25 is higher. Therefore, the differential driving can provide a larger extinction ratio. The extinction ratio is a ratio of a maximum value to a minimum value of transmission light intensity that are obtained when the transmission light intensity of an optical modulator is varied, and serves as a barometer of the performance of the optical modulator.
The cutoff frequency fc (single phase) of the EA modulator 25 in the case of single-phase driving is given by Equation (1). The cutoff frequency is a frequency above which light cannot travel through a waveguide.fc(single phase)=1/(2πCR)  (1) where C is the capacitance of the EA modulator 25 and R is the matching resistance.
On the other hand, in the case of differential driving, the matching resistance is 2 R (50 Ω+50 Ω), which is two times greater than in the case of single-phase driving. Therefore, the cutoff frequency fc (differential) of the EA modulator 25 in the case of differential driving is given by Equation (2).fc(differential)=0.5/(2πCR)=0.5 fc(single phase)  (2) 
That is, there is a problem that the cutoff frequency fc (differential) in the case of differential driving is as low as a half of the cutoff frequency fc (single phase) in the case of single-phase driving.